Introduction

On September 28, 2012, the US Food and Drug Administration approved the totally subcutaneous implantable cardioverter defibrillator (S-ICD) for the treatment of life-threatening ventricular tachyarrhythmias in patients who do not have symptomatic bradycardia, incessant ventricular tachycardia (VT), or spontaneous, frequently recurring VT that is reliably terminated with antitachycardia pacing (ATP). These devices hold the promise of life-saving defibrillation without some of the known problems of transvenous defibrillator systems (TV-ICD), including limited vascular access, transvenous lead durability, hazardous lead extractions, and risks of bloodstream infections. There are trade-offs with the S-ICD devices including reports of increased infections and the inability to deliver ATP. The subcutaneous position of the electrodes requires higher energy delivery with longer charge times. In addition, sophisticated signal processing is required for accurate arrhythmia detection and diagnosis. As we consider the appropriate patient population for S-ICD therapy, it is important to review the trade-offs of this system compared with traditional TV-ICD systems.

Response by Poole and Gold on p 1251

Detection of Ventricular Arrhythmias

Unlike transvenous ICDs that sense the local ventricular myocardial depolarization with an endocardial bipolar electrode, the S-ICD records a signal from 2 of its 3 subcutaneous electrodes. These are the distal sensing electrode located at the lead tip, a proximal sensing electrode located 14 cm proximal to the distal electrode, and the pulse generator itself. By their nature, the S-ICD signals are more susceptible to external and myopotential noise, as well as T-wave oversensing. Thus, sophisticated algorithms have been developed to filter noise from the true cardiac signals.1 Nevertheless, oversensing is responsible for the majority of inappropriate shocks in these systems as reported in a recent multicenter case–control study.2

The competing concern is that of undersensing true arrhythmias that can lead to delays in therapies. The best data available on S-ICD time to therapy come from …